Process of preparing copper foil for use in fine geometry circuit boards

ABSTRACT

Process of preparing copper foil for use in fine geometry circuit boards in which spaced apart islands are formed in the copper foil, with the islands being smaller in dimension than crystals in the foil so that most of the islands are single crystals, and the islands are grown out until they meet one another to form a continuous foil having a columnar structure that etches preferentially along the intersections of the islands in directions perpendicular to the surface of the foil thus formed.

BACKGROUND OF THE INVENTION

1. Field of Invention

This invention pertains generally to the manufacture of circuit boards and, more particularly, to a process of preparing copper foil for use in fine geometry circuit boards.

2. Related Art

Copper foil used in the manufacture of electronic circuit boards is typically electrodeposited on a carrier such as aluminum foil prior to being laminated to a substrate fabricated of an electrically insulative material such as fiberglass and epoxy. The carrier foil protects the copper foil during lamination and is not removed until just prior to the patterning and etching of the copper foil.

The copper foil may have a random crystal structure and imperfections that can cause etching to occur in an irregular manner. As a result, conductive lines or traces etched from the foil are often made wider than might otherwise be desired in order to ensure proper conductivity throughout their length even if irregularities do occur in the etching. The need for wider lines limits the ability to reduce the size of circuit boards and the equipment in which they are used.

OBJECTS AND SUMMARY OF THE INVENTION

It is, in general, an object of the invention to provide a new and improved process of preparing copper foil for use in printed circuit boards.

Another object of the invention is to provide a process of the above character which is particularly suitable for preparing copper foil for use in finer geometry circuit boards.

These and other objects are achieved in accordance with the invention by providing a process of preparing copper foil for use in fine geometry circuit boards in which spaced apart islands are formed in the copper foil, with the islands being smaller in dimension than crystals in the foil so that most of the islands are single crystals, and the islands are grown out until they meet one another to form a continuous foil having a columnar structure that etches preferentially along the intersections of the islands in directions perpendicular to the surface of the foil thus formed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of one embodiment of copper foil being prepared for use in fine geometry circuit boards in accordance with the invention.

FIG. 2 is a cross-sectional view taken along line 2-2 in FIG. 1 of the copper foil after the processing has been completed and the foil is ready for use in circuit boards.

DETAILED DESCRIPTION

Referring now to FIG. 1, the process begins with copper foil on a metal carrier 11. The copper foil is covered with photoresist (not shown) which is patterned to define a plurality of rectangular or square islands with dimensions that are small compared to the size of the crystals in the foil and to the thickness of the final foil. The exposed copper is then etched away, leaving just the islands of copper 12 with vertical side walls 13 on the carrier.

Although the original foil may consist of many crystals of different orientation, making the islands small compared to the size of the crystals ensures that the copper in most, if not all, of the individual islands will come from just one crystal and that each island will thus be a single crystal.

Following etching, the copper is grown by a suitable process such as electroplating until the islands grow together as illustrated in FIG. 2 to form a continuous foil. If the islands are single crystals, the only crystal imperfections should be vertical imperfections along the sides of the crystals where they meet. This is a columnar structure which should etch faster in a direction perpendicular to the surface than in other directions, which makes the foil uniquely suitable for use in circuit boards with fine lines and spaces.

If the islands are not single crystals, there may be crystal imperfections on the sides of the islands that can grow out in walls that are parallel to the copper foil surface and cause lateral etching at relatively high rates. Such lateral etching can result in undesired etching of the sides of conductor lines on a circuit board, and while some lateral etching can be tolerated, care should be taken to limit it by making the islands initially small in dimension relative to the crystals in the original foil.

As the islands grow out toward each other, they also grow in height until the final thickness of the foil is reached, and it is important that the islands not finish growing out to meet one another before the final thickness of the foil is reached. Otherwise, the intersections 14 between the islands will be plated over, and the top layer of the foil will not have the columnar structure. Instead, it will be composed of regions of single crystals whose dimensions may be much larger than the top surfaces of the columns. The upper portion of the foil will then etch more rapidly at the intersections of these regions, rather than along the intersections of the columns, and the edges of lines etched in the foil will be ragged.

If the islands or columns do not grow out to meet one another at the top of the foil, the top of the foil will have a waffle-like structure of indentations 16 which will provide an even more preferred etch rate perpendicular to the top surface of the foil.

The copper grows at substantially the same rate in all directions, and the point at which the islands grow together relative to the final thickness of the foil is determined by the dimensions of the original islands 12 and the distances between them. Thus, for example, the initial thickness or height of the islands should be sufficient to ensure that the intersections of the islands extended during growth will not be plated over when the foil comes to its final thickness.

The lines in a circuit board must have sufficient cross-sectional area to provide the desired conductivity, and to increase the line density, the height of the lines could be made greater than the width. That generally is not done, however, because of mechanical strength and the possibility of etching through the lines at crystal imperfections in the side walls. Instead, it is preferable to make the width of the lines greater than the height (e.g., twice the height) so that an occasional imperfection in width will not impair the performance of the lines. It is also preferable to make the spaces between the lines sufficiently large (e.g., twice the width of the lines) to prevent undesired shorting between the lines.

It is apparent from the foregoing that a new and improved process of preparing copper foil for use in fine geometry circuit boards has been provided. While only certain presently preferred embodiments have been described in detail, as will be apparent to those familiar with the art, certain changes and modifications can be made without departing from the scope of the invention as defined by the following claims. 

1. A process of preparing copper foil for use in fine geometry circuit boards, comprising the steps of: forming spaced apart islands in the copper foil, with the islands being smaller in dimension than crystals in the foil so that most of the islands are single crystals, and growing the islands out until they meet one another to form a continuous foil having a columnar structure that etches preferentially along the intersections of the islands in directions perpendicular to the surface of the foil thus formed.
 2. The process of claim 1 wherein the islands are also grown in height to the final thickness of the foil, and the islands do not finish growing out to meet one another before the final thickness of the foil is reached.
 3. The process of claim 1 wherein growing of the islands is terminated before the islands have grown completely together at the surface of the foil, leaving indentations in the top surface above the intersections of the islands which further promote etching perpendicular to the top surface of the foil. 